You can easily add them by following the instructions on their site.
On immutable fedora it can be done via
curl -o - https://repository.mullvad.net/rpm/stable/mullvad.repo | sudo tee /etc/yum.repos.d/mullvad.repo
rpm-ostree uninstall mullvad-vpn --install mullvad-vpn
# after reboot, if not working
sudo systemctl start mullvad-daemon
L e a r n h o w f l a t p a k i s a s e c u r i t y a n t i - p a t t e r n .
/s
Instructions uncleared, flatpak expended my dong